For the line-interactive UPS (Uninterruptible Power Supply) system, there are generally three operation modes: on-line mode, back-up power mode and bypass mode. During the back-up mode, a battery will supply a voltage passing through an inverter to a load and the inverter will convert a DC voltage from the battery into an AC voltage. Please refer to FIG. 2, which illustrates a conventional charging circuit in the line-interactive UPS system. During a normal operation (on-line mode), a relay 101 or a switch is electrically conducted by a main power 102 electrically connected to output terminals 103 and 104 so as to directly output an AC voltage to a load (not shown). Then, when a controller detects an abnormal output of an AC line voltage, namely, the main power 102 is abnormal, the relay 101 or the switch will cut off the conduction thereof, and further, a battery 105 will provide a DC voltage, which can be converted into an AC voltage by an inverter 106. Therewith, the AC voltage is transformed by a transformer 107 for outputting.
During the normal operation mode, the line-interactive UPS will directly output the AC output voltage from the main power 102 while the main power 102 simultaneously charges the battery through the transformer 107 and the inverter 106. In addition, the line-interactive UPS utilizes leakage inductors of the transformer 107 to form a boost converter for charging the battery, whose details are disclosed in U.S. Pat. No. 5,302,858.
However, because in this circuit, the two switching devices Q1 and Q2 do not participate in the charging operation, the two switching devices Q3 and Q4 have to be controlled to be conductive so that the transformer 107 can be shorted for a specific duration for rapidly charging the leakage inductors of the transformer 107. When the conduction of the switching devices Q3 and Q4 is cut off, the leakage inductors will generate a maintaining current and also charge the battery 105 through anti-parallel diodes. Therefore, according to the description above, it is obvious that the conventional line-interactive UPS needs a complex circuit for realizing the purpose of current control.
In order to overcome the drawbacks in the prior art, a charging circuit in uninterruptible power supply system is provided.